Technical Field
The present disclosure relates to an organic light emitting diode display.
Discussion of the Related Art
Various display devices have replaced heavier and larger cathode ray tubes (CRTs). Examples of the display devices may include a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), and an organic light emitting diode (OLED) display.
In more detail, an OLED display is a self-emission display configured to emit light by exciting an organic compound. The OLED display does not require a backlight unit used in a liquid crystal display and thus has advantages of a thin profile, lightness in weight, and a simpler manufacturing process. The OLED display can be also manufactured at a low temperature and has a fast response time of 1 ms or less, low power consumption, a wide viewing angle, and a high contrast. Thus, the OLED display has been widely used.
The OLED display includes organic light emitting diodes (OLEDs) converting electric energy into light energy. The OLED includes an anode, a cathode, and an organic emission layer between the anode and the cathode. The OLED display is configured such that the OLED emits light while excitons formed by combining holes from the anode and electrons from the cathode inside an emission layer fall from an excited state to a ground state, and thus displays an image.
However, a large-area OLED display cannot maintain a uniform luminance throughout an entire surface of an active area, on which an input image is displayed, and generates a luminance variation (or luminance deviation) depending on a position. More specifically, a cathode constituting an organic light emitting diode is formed to cover most of the active area, and there is a problem that a power voltage applied to the cathode does not have a constant voltage value throughout the entire surface of the active area. For example, as a difference between a voltage value at an entrance of the cathode supplied with the power voltage and a voltage value at a position apart from the entrance increases due to a resistance of the cathode, the luminance variation depending on the position increases.
The problem is more problematic in a top emission type display device. Namely, in the top emission type display device, because it is necessary to secure a transmittance of a cathode positioned at an upper layer of an organic light emitting diode, the cathode is formed of a transparent conductive material such as indium tin oxide (ITO), or an opaque conductive material with a very small thickness. In this instance, because a surface resistance of the cathode increases, a luminance variation depending on a position remarkably increases corresponding to an increase in the surface resistance.
In order to solve such a problem, a method was proposed to prevent a voltage drop depending on a position by forming a low potential power voltage line including a low resistance material and connecting the low potential power voltage line to a cathode. In the proposed method according to a related art, because the low potential power voltage line was formed on a lower substrate including transistors, one pixel has to further include a connection area of the low potential power voltage line and the cathode in addition to a thin film transistor area and a storage capacitor area. Thus, it was difficult to apply the related art to a high-resolution display including small-sized unit pixels.